1. Field of the Invention
The present invention relates to off-axis reinforced symmetric and asymmetric shapes and methods for forming symmetric and asymmetric shapes with off-axis reinforced seamless woven structures.
2. Background of the Invention
Radomes are housings used in the aerospace industry, which may shelter an antenna, radar, or similar devices from the surrounding environment. For example, a radome may be used to protect an antenna from high winds, icing, and/or temperature extremes in an area surrounding the device. Without protection, these devices become vulnerable to the adverse effects of rain, heat, erosion, pressure, and other sources of damage, depending upon where the device is used.
Radomes for aerospace applications have typically been of axisymmetric or symmetric shapes usually in the form of a cone, hemisphere, paraboloid of revolution, or half ellipsoid of revolution. However, radomes of these shapes, fail to meet the radar cross-section (RCS) requirements imposed by some government agencies. That is, although prior art radomes may adequately shelter the device assembly, because of their geometric shape, they have a high RCS and thus can be easily detected by radar. Unfortunately, radar-absorbing materials may not generally be used in conjunction with radomes because these materials may interfere with the broadcasting and reception of signals by components housed within the radome. For instance, in some aerospace applications, a radome protects the enclosed devices from aerodynamic forces and meteorological damage, while at the same time allowing radar transmission and reception, as well as preventing the devices from upsetting the aerodynamic characteristics of the airborne vehicle upon which it is mounted. Interference with signals transmitted to or from such devices would render the devices useless.
Another design problem facing the aerospace industry regarding radomes is that the materials used to construct a radome must be of sufficiently low dielectric properties so as not to interfere with transmitted signals. Additionally, the dielectric properties of the material must be uniform because non-uniform dielectric properties adversely effect transmissions entering or leaving the radome. For example, radomes are typically composite structures. These composite structures usually consist of glass or quartz fiber preforms with a matrix material that is a polymer having low dielectric properties. Radomes of this type may be constructed using resin transfer molding or by hand laying of glass fiber prepregs. The term “prepreg” is used to describe a fabric or uni-directional tape that is pre-impregnated with a polymeric resin. The construction of preforms from resin transfer molded parts may be done, for example, via cutting and stitching individual plies of glass fiber, shaped weaving, polar weaving and Jacquard weaving (shuttle loom). However, these techniques are time consuming and have a significant drawback in that the radome produced by such methods will result in at least one seam which is often the source of failure of such structures when subject to extreme conditions. Seams are also undesirable in radomes because the dielectric characteristics become non-uniform in the area of the seam and therefore can adversely effect signal transmission and reception.
Three-dimensional Jacquard weaving techniques may, however, be used in the construction of radomes. With these techniques, it is possible to construct a sock-like preform from one piece of cloth. This type of sock-like preform is advantageous since it is seamless, thus eliminating the significant problems associated with seams. In the basic weaving configuration, the warp and fill fibers are at right angles to one another so the continuous filling fiber is oriented in the hoop (90°) direction and the warp fibers follow axial contours (the 0° direction) on the final structure. Accordingly, a three-dimensional Jacquard woven, seamless sock preform is preferred over other techniques.
By selectively including or excluding part of the warp fiber, these socks may form various geometric shapes with or without closed ends. They may be woven as a single layer of fabric, however, multiple socks may be formed over one another to build up the desired thickness. The preform may then be processed into a composite structure using the previously discussed manufacturing techniques. However, weaving complex three-dimensional shapes, such as asymmetrical shapes, with current weaving techniques is a challenge.
In an effort to improve the stealth characteristics of aerospace vehicles, it has been shown that asymmetrically-shaped radomes are preferred since they improve the radar avoidance capabilities of the vehicle. However, the advent of asymmetrically-shaped radomes presents further challenges in the construction of such complex three-dimensionally shaped structures. A proposed solution is the use of automated tape laying techniques, whereby strips of glass (or quartz) prepregs are laid down onto a male mandrel. The tape laying head must have many degrees of freedom, apply consistent pressure and be capable of placing tapes accurately side by side. While such machines are available, they are extremely expensive.
In addition, radomes may need to be reinforced in the off-axis direction. For example, there are radome applications that require torsional strength or stiffness, preferably near the ±45° direction. Moreover, other applications that require bearing strength also utilize reinforcement in the ±45° directions along with reinforcement in the 0° and 90° directions. Off-axis reinforcement cannot be achieved with current basic weaving techniques because with current techniques, the warp and fill fibers are at right angles to one another so the continuous filling fiber is oriented in the hoop (90°) direction and the warp fibers follow axial contours (the 0° direction) on the final structure. Therefore, reinforcement along the ±45° directions is not achieved.
There are two conventional approaches for adding off-axis reinforcement. One approach is to cut two-dimensional cloth on a bias. These off-axis plies require significant handwork, and introduce seams that do not preserve the tube-like integrity of the 0°/90° sock and, as previously stated, adversely effect the structural integrity and dielectric properties of the radome. However, these off-axis plies may be used for very large preforms. The second approach is to use a braided sock. This approach preserves the tube-like nature of the preform, but there is a limit to the size of the preform. Further, this approach does not provide true hoop (90°) reinforcement.
Therefore, a need exists for a cost effective method for forming symmetrically and asymmetrically-shaped objects with off-axis reinforcement.